The ABO blood group system

Download PDF

In the ABO blood group system, the red blood cells in humans have molecular differences from individual to individual. The differences are systematic and can be characterized according to a system of four different hereditary types; A, B, AB and O. These types together form the blood grouping system, ABO.

The differences in these four hereditary blood types are due to the four types having different variants or combinations of antigens. In the ABO group the antigens are carbohydrate chains in the membrane on the surface of the red blood cells. Each human individual has antibodies to the variants that it does not have.

The carbohydrate chains are available in two main groups, A and B. Humans with blood type A, have type A carbohydrate chains. Humans with blood type B have type B carbohydrate chains. Humans of type AB have both chain types, while humans with type O do not have any of them. In plasma, a human being always has antibodies to the A- and/or B-antigen that it does not have. A person of blood type O has both anti-A and anti-B, a person of blood type A has anti-B, a person of blood type B has anti-A, whereas a person of blood type AB does not have any A or B antibodies.

The genome that determines the blood type in the ABO system is located on chromosome 9, and there are three alleles A, B and O. The gene codes for an enzyme (glycosyl transferase) that connects an additional carbohydrate unit to H-antigen (oligosaccharide) on red blood cells. The B allele provides an enzyme that connects to galactose. The A allele provides an enzyme that connects to N-acetyl-galactosamine while the O allele provides a non-functional enzyme.In the ABO blood group system, the red blood cells in humans have molecular differences from individual to individual. The differences are systematic and can be characterized according to a system of four different hereditary types; A, B, AB and O. These types together form the blood grouping system, ABO.

The differences in these four hereditary blood types are due to the four types having different variants or combinations of antigens. In the ABO group the antigens are carbohydrate chains in the membrane on the surface of the red blood cells. Each human individual has antibodies to the variants that it does not have.

The carbohydrate chains are available in two main groups, A and B. Humans with blood type A, have type A carbohydrate chains. Humans with blood type B have type B carbohydrate chains. Humans of type AB have both chain types, while humans with type O do not have any of them. In plasma, a human being always has antibodies to the A- and/or B-antigen that it does not have. A person of blood type O has both anti-A and anti-B, a person of blood type A has anti-B, a person of blood type B has anti-A, whereas a person of blood type AB does not have any A or B antibodies.

The genome that determines the blood type in the ABO system is located on chromosome 9, and there are three alleles A, B and O. The gene codes for an enzyme (glycosyl transferase) that connects an additional carbohydrate unit to H-antigen (oligosaccharide) on red blood cells. The B allele provides an enzyme that connects to galactose. The A allele provides an enzyme that connects to N-acetyl-galactosamine while the O allele provides a non-functional enzyme.